Electron gun for providing electrons grouped in short pulses

ABSTRACT

This gun comprises a cathode K, a grid G, and an anode A between which the applied voltages are radio-frequency voltages. The cathode is disposed on the central conductor of a coaxial cavity, facing said grid terminating said cavity. Said cavity is terminated at the other end by a short-circuit and includes a coaxial branch line so as to resonate at two frequencies F 1  and F 2  multiple of f 0 , whose beating induces a radio-frequency grid-cathode voltage. Said grid terminates another coaxial cavity whose central conductor is hollow and whose end facing said grid forms the anode. said other coaxial cavity resonator is excited and resonates at a frequency F 0  multiple of f 0 , which induces a radio-frequency anode-grid voltage. 
     A proper selection of the frequencies F 0 , F 1 , F 2  allows to obtain electrons bunches of very short duration.

BACKGROUND OF THE INVENTION

The present invention relates to an electron gun for providing electronsgrouped in short pulses of predetermined pulse repetition frequency f₀.

In many applications, it is necessary to provide electrons grouped inshort bunches. This is in particular the case when it is desired toinject these electron bunches into accelerating systems of thehigh-energy linear type.

The conventional solutions use electron guns with a triode structureformed by an electron-emitting cathode, a grid and an anode, allaligned. The electrons are provided during the times where a gatingvoltage is applied to the grid, the anode and the cathode being suppliedwith DC voltages.

A major disadvantage of this approach is related to the gating of thegrid during a very short time, for example shorter than a nanosecond. Asa matter of fact, the presence of inevitable parasitic capacitancesproduces in the triggering circuits time constants which are difficultto decrease. If, in addition, as is the case in certain applications, itis desired to obtain electrons grouped in extremely short times, ofabout 10 to 100 picoseconds, it is necessary to effect a velocitymodulation with an additional cavity resonator, which increases thecomplexity and the cost of the device.

SUMMARY OF THE INVENTION

A purpose of the present invention is to remedy these disadvantagesthanks to a very simple solution allowing to eliminate the usualtriggering circuits.

An object of the present invention is an electron gun in which all thevoltages being used are radio-frequency voltages. By radio frequency, itis understood, in accordance with common usage, frequencies higher thana few tens of kilohertz.

According to the present invention, it is thus provided an electron gunto provide electrons grouped in short pulses of predetermined pulserepetition frequency f₀, said gun comprising a triode structure formedof an electron-emitting cathode, a grid and an anode, comprising firstmeans to generate a radio-frequency cathode-grid voltage from at leastone radio-frequency wave of frequency at least equal to said pulserepetition frequency f₀, and second means for generating aradio-frequency anode-grid voltage from a first radio-frequency wave offrequency F₀ =k₀ f₀, where k₀ is an integer equal to or greater than 1.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood and other features andadvantages will become apparent from the following detailed descriptionof a preferred embodiment given as a non-limitative example withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an electron gun according to thepresent invention without its power supply device;

FIG. 2 is a block diagram of the electron gun according to the inventionincluding the power supply circuits;

FIG. 3 shows curves representing the various voltages as a function oftime for an example of selected frequencies;

FIG. 4 shows similar curves for another set of frequencies; and

FIG. 5 shows the curves of FIG. 3 for an optimized embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a schematic of the structure of an electron gunaccording to the present invention is shown.

The purpose of this gun is to provide electrons grouped in short pulseswith a pulse repetition frequency f₀.

This gun comprises in a known manner an electron-emitting cathode K, agrid G and an anode A.

Instead of applying to the cathode and to the anode DC voltages, and tothe grid gating pulses of short duration, according to the inventionthere provided to apply between the grid and the cathode, for one thing,and between the grid and the anode, for another, radio-frequency ACvoltages. To this end, a first coaxial cavity resonator 1 including acentral conductor 2 is provided. This cavity resonator is terminated atone end by a short-circuit 3 and is terminated at the other end by acathode-grid capacitance KG, the grid G delimiting the cavity resonator1, and the cathode K being carried at the end of the central conductor 2facing the grid G. On the cavity 1, a coaxial branch line 4 with acentral conductor 5 is provided. This branch line is terminated by ashort-circuit 6 disposed so that the length of the branch line is equalto λg₁ /4, where λg₁ is the wavelength in the coaxial cavity resonator 1corresponding to a frequency F₁ =k₁ f₀ at which the cavity 1 is designedto resonate (as a function of its size and of the capacitance KG), k₁being an integer equal to or greater than 1. At this frequency, thebranch line 4 produces an infinite impedance on the cavity resonator 1and consequently does not affect it. On the other hand, this branch lineallows to have the cavity resonator 1 resonate at a second frequency F₂=k₂ f₀, where k₂ is an integer greater than 1 such that k₂ =pk₁, with pbeing an integer greater than 1.

For example, iF the branch line 4 is located substantially in the middleof the cavity resonator 1 and if it is assumed that p=2, the cavityresonator 1 will resonate at the frequency F₁ and at the frequency F₂,which is twice F₁, the branch line 4 producing at the frequency F₂ ashort-circuit in the coaxial structure 1 due to its length equal to λg₂/2.

The excitation waves with radio frequencies F1 and F2 are applied to thecavity 1 through excitation inputs 7 and 8.

Thus, there is obtained in the cavity 1 a radio frequency wave resultingfrom the beating of the waves of frequencies F₁ and F₂ and inducing aradio-frequency voltage between the cathode and the grid.

In a similar manner, a second coaxial cavity resonator 10 including acentral conductor 11 is provided. This cavity resonator 10 is terminatedat one end by a short-circuit 17 and is terminated at the other end by agrid-anode capacitance GA, the grid G delimiting the cavity 10, and theanode A being formed by the end of the central conductor 11 facing thegrid G. The central conductor is comprised of a hollow cylinder whoseinner space will allow the passage of the electron bunches emitted alongthe axis 15 of the assembly, as will be seen below. The characteristicsof the cavity resonator 10 and the capacitance GA are determined so thatthe cavity 10 resonates at a frequency F₀ =k₀ F₀, where k₀ is an integerequal to or greater than 1.

The exciting wave at the radio frequency F₀ is applied to the cavityresonator 10 through an excitation input 13.

There is thus obtained in the cavity resonator 10 a radio-frequency waveat the frequency F₀ which induces a radio-frequency voltage between thegrid and the anode.

Dielectric supports 16 may be provided to ensure a better supporting andcentering of the central conductor 11. Furthermore, there is provided atthe end of the inner space of the central conductor 11 a window 14 forthe passage of the electrons.

Finally, a solenoid 12 surrounds the cavity resonator 10 over the lengthof the central conductor 11 to focuse the electrons along the axis 15and thus form a drift space within this conductor.

The operation of the device will be better understood by means of thecurves of FIG. 3 which correspond to an example in which the followingvalues have been selected:

    k.sub.0 =1 (F.sub.0 =f.sub.0), k.sub.1 =4 (F.sub.1 =4f.sub.0)

and

    k.sub.2 =8 (p=2, F.sub.2 =8f.sub.0).

In the diagram of FIG. 3, the voltages V are shown as a function oftime, T₀ being the repetition period of the electron bunches (T₀ =1/f₀).The curve VF₀ represents the anode-grid voltage. The curve VF_(b)represents the grid-cathode voltage resulting from the beating of thetwo frequencies F₁ and F₂ represented in FIG. 3 by the curves VF₁ andVF₂ assuming that the amplitudes of the two waves are equal.

The current of electrons passes only when the grid-cathode voltageVF_(b) and the anode-grid voltage VF₀ are simultaneously positive. Itshould be noted here that the amplitude ratio of these two voltages isabsolutely not reproduced in the Figure to allow their representation ina single diagram. For example, the power injected into the cavity 10 maybe of about 30 kW, which corresponds to voltages of a few tens to aboutone hundred kilovolts, while the powers injected into the cavity 1 maybe of about 50 W each, which corresponds to voltages of a few hundredvolts.

Thus, in FIG. 3, the electron current will pass only during the hatchedpositive peak of VF_(b). The other peaks of VF_(b) will either produceonly a very little accelerated current of electrons, easily eliminated,corresponding to a substantially zero value of VF₀, or will give nocurrent of electrons since the latter will be blocked by a very negativeanode-grid voltage VF₀. Thus, in each period of the voltage VF₀, at thefrequency f₀, a bunch of electrons will pass only for a short timecorresponding to the width of the peak of VF₀.

For example, if a Frequency F₀ of 62.5 MHz, that is a period T₀ =16 ns,is selected, the current of electrons will pass only For 1 ns, when thevoltage VF₀ is maximum.

A very simple means is thus available for obtaining a pulse of 1-nsduration containing charges of about 4 nano-coulombs, For example, witha cathode delivering 4 amperes.

Power supply to the cavity resonators 1 and 10 can easily beimplemented, for example by a circuit such as that of FIG. 2.

In this Figure, an oscillator source 20 provides directly a wave at thefrequency F₀ =F₀ which is applied to the excitation input 13 of thecavity 10 after passing through an amplitude adjusting device 21,possibly in a phase adjusting device 21', although this is notindispensable in every case, and an amplifier 22 for the signal appliedto the input 13 to have the desired phase and amplitude.

Furthermore, a coupler 23 allows to tap a portion of the energy providedby the source 20 to send it to the inputs 7 and 8 of the cavity 1. Thistapped energy is sent, for one thing, to the input 7 after passingthrough a frequency multiplier, multiplying here by 4, to obtain thefrequency F₁ =4F₀, through an amplitude adjusting device 25, a phaseadjusting device 26, and an amplifier 27.

Finally, a coupler 28 taps a portion of the energy the output of thefrequency multiplier 24 to send it to the input 8 after passing througha frequency multiplier 29, multiplying here by 2, to obtain thefrequency F₂ =8f₀, through an amplitude adjusting device 30, a phaseadjusting device 31, and an amplifier 32.

The advantage of providing a single power supply source 20 is that it isnot necessary to use complex circuits for feedback control of the phaseand frequency of the various radio-frequency waves being used.

Referring to FIG. 4, there are shown curves corresponding to anotherselection of frequencies which allows to better appreciate the variouspossible solutions.

In the case of FIG. 4, the values k₀ =4 (hence F₀ =4f₀), k₁ =1 (hence F₁=f₀) and k₂ =4 (hence F₂ =4f₀) have been selected. The same curves wouldbe obtained if, for reasons of ease of implementation of the variouscoaxial structures and supply circuits, frequencies multiple of thesevalues, for example F₀ =8f₀, F₁ =2f₀ and F₂ =8f₀, had been selected.

In the example of FIG. 4, it has been chosen to excite a wave at thefrequency F₂ with an amplitude half that of the wave at the frequencyF₁.

It can be seen that there is effectively obtained a main peak 40 of thecurve VF_(b) whose hachured portion corresponds to the passage of anelectron beam and which repeats at the desired frequency f₀. But herethis peak is flanked by secondary peaks 41 and 42 of the curve VFb,which also give rise to a passage of an electron beam in their hachuredportion coinciding with a positive anode-grid voltage. These secondarybeams are undesired. Moreover, the main electcon beam corresponding tothe pic 40 is wider that in the case of FIG. 3. If nonetheless thissolution illustrated in FIG. 4 is chosen for other reasons, it ispossible to eliminate the effect of the secondary peaks by applying a DCadditional bias to the grid G, which shifts the levels of curve VF_(b).

Furthermore, this FIG. 4 allows to illustrate that it is possible todecrease the relative amplitude of the secondary peaks with respect tothe main peak by selecting a higher ratio between the amplitudes of thewaves at the frequencies F₁ and F₂.

When considering FIG. 3, the reason can also be seen for which it hasbeen chosen to create a beat between two frequencies F₁ and F₂ in thecathode-grid cavity 1. There are thus obtained narrower peaks of thevoltage VF_(b), hence electrons grouped in a shorter pulse than if onlythe frequency F₁ had been used, while substantially reducing theemission of undesired electrons.

The selected values corresponding to FIG. 3 represent an interestingtradeoff.

Referring to FIG. 5, another important aspect of the present inventionis shown. In this FIG. 5, there are shown the curves VF_(b) and VF₀corresponding to the same selection of frequencies as in FIG. 3, onlythe amplitude ratio at the frequencies F₁ and F₂ passing from 1 to 2 bysimple way of example.

The significant difference with FIG. 3 is that the voltage VF_(b) isphase-shifted with respect to the voltage VF₀ by a quantity equal tohalf the phase width of the electron bunch at the anode, i.e.,substantially half the width of the peak of VF_(b) (this width is hereof about 22°). In this case, the first electron will pass the anode whenan anode-grid voltage substantially equal to V₀ cos 22° is applied,where V₀ is the maximum value of the anode-grid voltage VF₀. As theelectrons pass, the anode-grid voltage accelerating these electrons willincrease up to the value V₀ for the last passing electron. Thanks tothis difference in accelerating voltage applied to the variouselectrons, at the end or a drift space or adequate length, asignificantly improved grouping or the bunch or electrons is obtained.Thus, in the numerical example given above (f₀ =62.5 MHz; duration orthe electron bunch at the anode or about 1 ns), assuming that thevoltage V₀ =80 kV and the drift length is or about 1 m, the duration orthe electron bunch is reduced to about 100 ps. This result can befurther improved by the addition or bunching cavities inserted at theend or the drift space, which allows to obtain pulses (bunch duration)or about 10 ps. The phase shirt or the voltage VF_(b) with respect tothe voltage VF₀ is easily obtained by means or the phase adjusting means26 and 31 (FIG. 2).

In the example which has been described with reference to FIG. 1, abranch line with a length λg₁ /4 has been provided to have the cavity Iresonate at two different frequencies. But it is clear that it ispossible to use any other known equivalent means disposed according tothe selected frequency ratios.

Thanks to the use or coaxial cavities and or radio frequency powersupplies, there is obtained, with a simple implementation, an electrongun in which the modulation problems associated with the use of shortgating DO voltage pulses (under one nanosecond) are eliminated.

It will be appreciated that the examplary embodiments described here arein no way limitative or the present invention.

What is claimed is:
 1. An electron gun to provide electrons grouped inshort pulses with a predetermined pulse repetition frequency f₀, saidelectron gun comprising:a triode structure made up of anelectron-emitting cathode K, a grid G and an anode A, comprising; firstmeans to generate a radio-frequency first voltage difference between thecathode and the grid from at least one radio-frequency wave of frequencyat least equal to said pulse repetition frequency f₀, wherein allcurrent in the cathode is generated by said radio-frequency firstvoltage, and second means to generate a radio-frequency second voltagedifference between the grid and the anode from a first radio-frequencywave of frequency F₀ =k₀ f₀, wherein k₀ is an integer equal to orgreater than
 1. 2. An electron gun according to claim 1, wherein saidfirst voltage is generated from the beating of a second and a thirdradio-frequency waves of respective frequencies F₁ =k₁ f₀ and F₂ =k₂ f₀,where k₁ and k₂ are integers such that k₂ =pk₁, with p being an integergreater than 1, and k₁ being equal to or greater than
 1. 3. An electrongun to provide electrons grouped in short pulses with a predeterminedpulse repetition frequency f₀, said electron gun comprising:a triodestructure made up of an electron-emitting cathode K, a grid G, and ananode A, comprising: first means to generate a radio-frequency firstvoltage difference between the cathode and the grid from at least oneradio-frequency wave of frequency at least equal to said pulserepetition frequency f₀ ; second means to generate a radio-frequencysecond voltage difference between the grid and the anode from a firstradio-frequency wave of frequency F₀ =k₀ f₀, where k₀ is an integerequal to or greater than 1; wherein said first voltage difference isgenerated from the beating of a second radio-frequency wave with a thirdradio-frequency wave whose respective frequencies are F₁ =k₁ f₀ and F₂=k₂ f₀, wherein k₁ and k₂ are integers such that k2=pk₁, with p being aninteger greater than 1, and k₁ being equal to or greater than 1; andwherein said first means comprises a first coaxial cavity resonatorhaving a central conductor one end of which is terminated by ashort-circuit and whose other end is terminated by said grid, saidcathode disposed at the end of said central conductor facing said gridto form which it a first capacitance terminating said first cavityresonator, and wherein the characteristics of said first coaxial cavityresonator are selected so that it resonates at said second frequency F₁further comprising a third means disposed on said first cavityresonator, for resonating at said third frequency F₂, said first cavityresonator comprising two excitation inputs fed respectively by the tworadio-frequency waves at the frequencies F₁ and F₂.
 4. An electron gunaccording to claim 3, wherein said third means comprises a coaxialbranch line terminated by a short-circuit disposed so that the length ofsaid branch line is equal to (2q +1) λg₁ /4, where λg₁ is the wavelengthcorresponding to said second frequency f₁, and where q is an integerequal to or greater than
 0. 5. An electron gun according to claim 3,wherein said first means comprises fourth means for generating aradio-frequency wave at said second frequency F₁ and a radio-frequencywave at said third frequency F₂, and for applying them to saidexcitation inputs with predetermined phases and amplitudes.
 6. Anelectron gun according to claim 5, wherein said fourth means comprises:aradio-frequency oscillator source; two power supply channelsrespectively connected to said excitation inputs of the first cavityresonator and each including an amplitude adjusting device, a phaseadjusting device and an amplifying device, as well as a frequencymultiplier in at least one of said channels; and at least one coupler toconnect the output of said radio-frequency oscillator source to said twopower supply channels.
 7. An electron gun according to any one of claims3-6, wherein said second means comprises a coaxial cavity resonatorhaving a central conductor one end of which is terminated by ashort-circuit and whose other end is terminated by said grid, said anodebeing formed by the end of said central conductor facing said grid toform with it a second capacitance GA terminating said second cavityresonator, and said central conductor being formed by a hollow cylinderwhose inner space allows the passage of the electrons emitted along theaxis of said first and second cavity resonators, wherein a focusingsolenoid surrounds said second cavity resonator over the length of saidcentral conductor to form a drift space within the latter, and whereinthe characteristics of said second coaxial cavity resonator and saidgrid-anode capacitance GA are selected so that said cavity resonates atsaid frequency F₀, said second cavity including an excitation input fedby said first radio-frequency wave.
 8. An electron gun according toclaim 7, wherein said second means comprise in addition fifth means togenerate said first radio-frequency wave at said frequency F₀ and toapply it to said excitation input of the second cavity resonator with apredetermined phase and amplitude.
 9. An electron gun according to claim8, wherein said fifth means comprises:a radio-frequency oscillatorsource; and a power supply channel connecting said oscillator source tosaid excitation input through an amplitude adjusting device, and anamplifying device.
 10. An electron gun according to claim 1, whereinsaid second means comprises a drift space, and further comprising;abunching cavity resonator disposed along an electron drift direction ofsaid drift space.
 11. An electron gun according to claim 6, wherein saidphase adjusting devices are adjusted so that said second and thirdradio-frequency waves are in phase, and so that the phase of theresulting beat wave is phase-shifted with respect to said firstradio-frequency wave by a quantity such that said second voltage isincreasing during passage of the electrons of a pulse through saidanode.
 12. A device according to claim 9, wherein said fifth meansfurther comprises:a phase adjusting device for adjusting the phase of asignal supplied by said power supply channel.
 13. An electron gun inwhich all applied voltages are above 20 kilohertz for providingelectrons grouped in short pulses with a predetermined pulse repetitionfrequency f₀, said gun comprising:a triode structure made of anelectron-emitting cathode K, a grid G and an anode A, comprising: firstmeans to generate a radio-frequency first voltage difference between thecathode and the grid from at least one radio-frequency wave of frequencyat least equal to said pulse repetition frequency f₀ ; and second meansto generate a radio-frequency second voltage difference between the gridand the anode from a first radio-frequency wave of frequency F₀ =k₀ f₀,where k₀ is an integer equal to or greater than
 1. 14. An electron gunfor providing electrons grouped in short pulses with a predeterminedpulse repetition frequency f₀, said electron gun comprising:a triodestructure comprising an electron-emitting cathode K, a grid G, and ananode A and voltage application means for applying only voltages greaterthan 20 kilohertz to the triode, wherein said voltage application meansapplies to said triode all voltages necessary for operation of thetriode, said voltage applying means comprising: first means to generatea radio-frequency first voltage difference between the cathode and thegrid from at least one radio-frequency wave of frequency at least equalto said pulse repetition frequency f₀ ; and second means to generate aradio-frequency second voltage difference between the grid and the anodefrom a first radio-frequency wave of frequency F₀ =k₀ f₀, where k₀ is aninteger equal to or greater than
 1. 15. An electron gun to provideelectrons grouped in short pulses with a predetermined pulse repetitionfrequency f₀, said electron gun comprising:a triode structure made up ofan electron-emitting cathode K, a grid G and an anode A, comprising;first means to generate a radio-frequency first voltage differencebetween the cathode and the grid from at least one radio-frequency waveof frequency at least equal to said pulse repetition frequency f₀,wherein all voltage between the cathode and the grid is due to saidradio-frequency first voltage, and second means to generate aradio-frequency second voltage difference between the grid and the anodefrom a first radio-frequency wave of frequency F₀ =k₀ f₀, wherein k₀ isan integer equal to or greater than
 1. 16. An electron gun according toclaim 15, wherein said first voltage is generated from the beating of asecond and a third radio-frequency waves of respective frequencies F₁=k₁ f₀ and F₂ =k₂ f₀, where k₁ and k₂ are integers such that k₂ =pk₁,with p being an integer greater than 1, and k₁ being equal to or greaterthan
 1. 17. An electron gun according to claim 15, wherein said secondmeans comprises a drift space, and further comprising;a bunching cavityresonator disposed along an electron drift direction of said driftspace.